Page 916 - The Veterinary Care of the Horse
P. 916

•    demonstrating aggressive behaviour

        •    interest in mares
  VetBooks.ir  •  attempting to cover mares in season

        •
             stallion-like appearance, e.g. a well-developed crest.



        Although retained testicles produce normal amounts of male hormone, they are incapable of
        producing sperm due to the body heat of the horse. Thus a bilateral rig is likely to be infertile,

        whilst  the  unilateral  rig  is  usually  fertile  since  the  descended  testicle  will  be  functioning
        normally.  There  are  stallions  that  have  bred  successfully  as  unilateral  rigs,  but  this  is
        controversial. Most vets advise against it because cryptorchidism is an inherited trait and so

        by breeding from them, there is a considerable chance of producing more defective progeny.
        One  USA  study  that  looked  at  more  than  5000  cryptorchid  horses  noted  an  increased

        incidence  in  some  breeds.  The  highest  incidence  was  found  in  Percherons,  American
        Saddlebred  horses,  Quarter  horses,  and  ponies  and  crossbred  horses.  However,  in  the

        Thoroughbred, Standardbred, Morgan, Tennessee Walking Horse and Arabian, the incidence
        was  less  than  would  have  been  expected  statistically.  This  may  have  been  a  reflection  of

        breeding policy.



        Diagnosis

        If the castration history of a particular animal is unknown and its behaviour gives cause for

        concern, blood tests can be used to determine whether a testicle is present.
        •    If the horse is over 3 years, the serum levels of oestrone sulphate are much higher in a

             horse with testicular tissue
        •    In younger horses, the testosterone levels are measured before and after an intravenous

             injection of human chorionic gonadotrophin (hCG). The testosterone level will rise 2-3

             times in a horse with testicular tissue and remain unaffected in geldings
        •    Anti-Müllerian hormone (AMH) is produced by descended and retained testicles. This

             blood test is useful for identifying cryptorchid horses that have had one testicle removed.
             It improves the accuracy of diagnosis when used in combination with either of the above

             tests. The serum AMH levels are higher in cryptorchid stallions compared with intact
             stallions; the levels are not detectable in geldings.

        •    Ultrasonography  of  the  inguinal  region  and  caudal  abdomen  can  be  helpful  in  the
             diagnosis of cryptorchidism and location of the retained testicle.



        WHERE IS THE MISSING TESTICLE?
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